1. Field of the Invention
The present invention pertains to electrothermally operated actuators of the type in which a piston is translatable in a piston housing in response to electrical heating of an expandable working fluid. In particular, the present invention pertains to an improved electrothermally operated actuator suitable for actuation of an operable device such as a valve.
2. Description of the Prior Art
There are many types of actuators for actuating operable devices: manual, hydraulic, pneumatic, electrical, or combinations thereof. Because of efficiency, compactness, remote controlling and other characteristics, electrically operated actuators are preferred in many situations. Current electric actuators, particularly of the linear type, utilize either solenoids or motor-gear trains rotating a ball screw to obtain linear motion of a threaded shaft.
Solenoid type actuators are typically used in applications such as water valves on dish washers and washing machines and numerous other applications in industry, office machinery, homes, vending machines, etc. However, solenoids are limited to operation of relatively small devices since the piston force produced is about one pound of force per cubic inch of solenoid volume.
A more expensive alternative to solenoid actuators are those which utilize a motor-gear train driving a ball screw. This type of actuator has a number of moving parts and is correspondingly less reliable than the solenoid. It is capable of producing larger forces for relatively large loads and is used where cost, weight and bulk are not problems. Obviously, the linear movement produced by an actuator of the motor-gear train type is much slower than that of a solenoid actuator.
In summary, the solenoid actuator has only one moving part, but low available force per unit volume. The motor-gear train actuator produces greater forces but is slower, more complex, relatively expensive, etc. Obviously, there is much room for improvement in electrically powered actuators, particularly in the area of moderate forces and speedy operation.
In the past, attempts have been made to pressurize a piston-cylinder actuator by electrically heating an expandable fluid in communication therewith. Examples may be seen in U.S. Pat. Nos. 2,322,762; 4,079,589; and 4,711,270. Most such efforts have been only moderately successful and have produced actuators too slow to compete with the solenoid actuator which has an operating time of a fraction of a second.
The present applicant, in a previous U.S. Pat. No. 4,235,413, disclosed an electrothermally operated actuator in which an expandable fluid, such as water, is heated in a tube and expanded into steam, the steam translating a piston within a cylinder or housing for transmitting force to an operable device, e.g. a valve. By controlling the supply voltage, the fluid heats up and actuates a valve from open to close in about twenty seconds. Equilibrium temperature is reached and the valve remains closed as long as the current is on. When the supply voltage is switched off, the fluid cools and condenses and the valve opens, under a spring load, in about twenty seconds. While this valve actuator is a substantial improvement over the prior art, its speed and power consumption could be improved.